Liver homeostasis is achieved by the removal of diseased and damages hepatocytes and their coordinated replacement to maintain a constant liver cell mass. Cirrhosis, viral hepatitis and toxic drug effects can all trigger apoptosis in the liver as a means to remove the unwanted cells, and the Fas 'death receptor'pathway comprises a major physiological mechanism by which this is occurs. The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor known to regulate both apoptotic and proliferative processes, and the AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is the prototype for a class of compounds known to affect these processes. Our long term goal is to understand mechanistically how the AhR contributes to tissue homeostasis by regulating cell growth and cell death. Our hypothesis, supported by the preliminary evidence, suggests that AhR activity sensitizes liver cells to Fas ligand (FasL) induced apoptosis, possibly by regulating expression of proteins that promote the cell death program. A plausible candidate is the AhR-regulated enzyme N-myristoyltransferase 2 (NMT2), because N-myristoylation of the Bid protein is critical for its activity in promoting FasL-induced apoptosis. The goal of this proposal is to study AhR function in the context of Fas-mediated liver apoptosis in vitro and in vivo. Aim 1 will examine whether the heightened susceptibility to Fas-mediated apoptotis depends on classical transcriptional activity by the AhR, or involves a non-classical mechanism. These studies will examine the severity of FasL-induced apoptosis in AhR-negative BP8 hepatoma cells expressing AhR molecules with targeted mutations that specifically disrupt AhR transcriptional activity. In Aim 2 we will determine whether the AhR-dependent susceptibility of hepatocytes to Fas-mediated apoptosis is due entirely to NMT2 action facilitating Bid activity. Aim 3 will examine the AhR's role in Fas-mediated apoptosis in isolated primary hepatocytes and in the liver in vivo. The studies will use an adenovirus gene transfer strategy to either express proteins, or use small interfering RNAs to suppress target gene expression in both cultured hepatic cells and the liver in vivo, in order to gain a mechanistic understanding of the functional relationship between the AhR and Fas-mediated hepatocyte apoptosis.